The present invention relates to a method of separating HF from HCl. The present invention also relates to a method of separating SiF.sub.4 from HCl.
More particularly the present invention relates to method of separating HF from HCl by converting the HF to SiF.sub.4 and subsequently separating the formed SiF.sub.4 from the HCl by distillation/absorption in an aqueous system.
Many industrial processes produce, as an unwanted by-product, a waste gas or liquid comprising HCl and HF. For example in the incineration of plastics, a weak aqueous solution of HCl and HF is obtained. A large quantity of HCl is produced as a waste by-product in the production of Freon. This waste HCl contains significant quantities of HF. Typically a portion of the HCl is recovered from the mixture by low temperature distillation. However, due to the difficulty of separating HCl from HF, a large amount of waste HCl contaminated with HF remains. In the past HCl containing HF has been sold as a very low grade HCl. The HF contamination in the HCl makes it unsuitable for many applications and the ultimate user must contend with the fact that HF is by definition a toxic substance.
There are many methods of treating HCl and HF to remove one component or the other. For example, Rectenwald el al U.S. Pat. No. 4,092,403 discusses a process of purifying hydrogen chloride containing HF by contacting the HCl with alumina to sorb fluoride compounds.
Gunn, Jr. U.S. Pat. No. 3,273,963 discloses removing HF from HCl by converting the HF to SiF.sub.4 and evaporating off the HCl in the presence of a suppressant. Further, Smith et al U.S. Pat. No. 2,526,776 discloses removing hydrogen fluoride from hydrogen chloride by passing the HF-HCl mixture over silica to form silicon tetrafluoride. Subsequently, the silicon tetrafluoride is sorbed on Fuller's Earth. Similar methods are disclosed in Sze U.S. Pat. No. 4,009,214 and Laird U.S. Pat. No. 2,343,462.
The primary method currently employed to remove hydrogen fluoride from HCl is to sorb it in calcium chloride or aluminum chloride as is disclosed in Robota et al, U.S. Pat. No. 4,317,805.
In general, although these references do disclose various separation techniques they are ineffective due to the low relative volatility of HF v. HCl and silicon tetrafluoride v. HCl. In fact, the Rectenwald reference teaches that silicon tetrafluoride is difficult to remove from HCl. It is for this reason that HCl contaminated with HF is sold as a very low grade acid with relatively little profit for the seller.